OCCURRENCE OF TINGUAITE AT KOSCIUSKO, N.S.W. 373 



V. Relation to allied rocks. — The foregoing table shows the 

 relation of the Kosciusko rock, from the point of view of its 

 chemical composition, to various nepheline-bearing rocks from 

 other parts of the world. 



It is obvious from the preceding table that (if we omit, on 

 account of its deficiency in potash, the phonolite of Purakanui 

 near Dunedin, New Zealand), there are three nepheline rocks in 

 the table to which the Kosciusko rock is, chemically, closely allied, 

 viz., (1) The nepheline-porphyry (Brogger) of the Lougenthal, 

 Christiania District. (2) the type of Laurdalite (Brogger) rich in 

 nepheline, from Lunde in the Lougenthal, and (3) the phonolitic 

 nephelinite of the Katzenbuckel in the Odenwald. The Kosciusko 

 rock is slightly poorer in alumina, lime and magnesia than the 

 two Lougenthal rocks, and is slightly more acid than the phonolitic 

 nephelinite of the Katzenbuckel which contains 48*24 per cent, of 

 silica, as compared with 51 J per cent, in the Kosciusko rock. 



Mineralogically the characteristics of these three rocks may be 

 summarised as follows :---(l) Nepheline porphyry of the Lougenthal 1 

 The rock consists of a medium grained ground-mass, with a per- 

 fect eugranitic structure with larger or smaller phenocrysts 

 (einspringlingen) up to 3 cm. in diameter of nepheline in stunted 

 prismatic crystals, bounded by the faces (0001) and (1010). Very 

 sparingly imbedded in the base are irregularly bounded poikilitic 

 plates of biotite and slightly idiomorphic individuals of felspar. 

 The ground-mass, which is composed of granules having an average 

 size of 2 to 5 mm., is formed of soda-orthoclase (cryptoperthite) 

 and orthoclase-microperthite, both of the same size and passing 

 into one another : it also contains nepheline and sodalite, but the 

 latter mineral occurs more sparingly, Amongst the darker 

 minerals are asgirinediopside with an outer zone of segirine, and 

 biotite (it may be lepidomelane) together with accessory titani- 

 ferous iron, pyrites, sphene and apatite. 



description abridged by us from Brogger' s "Die Eruptivgesteine des 

 Kristianiagebietes/' Vol. in., pp. 155 - 161. 



